1. Field of the Invention
The present invention relates to a communication master station startup period control method applied to a PLC system connecting a communication master station and a communication slave station with a bus type network.
2. Description of the Related Art
Conventionally, there is known a PLC system in which various pieces of data are exchanged between the communication master station and each communication slave station (refer to, for example, Japanese Patent Application Laid-Open No. 9-128019). In the PLC system, one programmable logic controller device (PLC device) having a communication facility and one or more I/O terminal devices having the communication facilities are connected with the bus type network, and the PLC device having the communication facility is set to the communication master station while each of the I/O terminal devices having the communication facilities is set to the communication slave station. In an operation state of the PLC device, a series of operations in which a response frame is returned to the communication master station is repeated by delaying timing of each of the corresponding communication slave station for a polling frame broadcast from the communication master station, which allows various pieces of data to be exchanged between the communication master station and each communication slave station. As used herein, “I/O terminal device” shall be a collective term of an input terminal device, an output terminal device, and an input and output terminal device.
There are various implementation modes in “the PLC device having the communication facility” which should be the communication master station. Examples of the implementation mode include the following items: (1) an implementation to which a communication master unit is attached in a building-block type PLC in which various functions can be added in unit, (2) an implementation in which the communication facility is incorporated into a CPU unit in itself in the building-block type PLC in which various functions can be added in unit, (3) an implementation into which the communication facility is further incorporated in an all-in-one type PLC in which all basic functions such as a computation process unit and an input and output unit necessary for PLC are accommodated in one housing, and (4) an implementation to which the communication master unit is externally attached in the all-in-one type PLC in which all the basic functions such as the computation process unit and the input and output unit necessary for PLC are accommodated in one housing. The building-block type PLC includes a type in which each unit is attached to a base board and a type in which units are attached to a DIN rail and the units are connected with a through connector provided in each unit
Similarly there are various implementation modes in “the I/O terminal device having the communication facility” which should be the communication slave station. Examples of the implementation mode include the following items: (1) an implementation into which the communication facility is incorporated in an all-in-one type I/O terminal device in which all basic functions such as a terminal stand and an input and output circuit necessary for the I/O terminal device are accommodated in one housing, (2) an implementation to which a communication slave unit is externally attached via cable in the all-in-one type I/O terminal device in which all the basic functions such as the terminal stand and the input and output circuit necessary for the I/O terminal device are accommodated in one housing, and (3) an implementation to which the communication slave unit is attached in a building-block type I/O terminal device in which the basic functions such as the terminal stand and the input and output circuit necessary for the I/O terminal device can be added in unit. In addition, (4) a repeater existing on the network can be included as long as the repeater has a node address.